In certain semiconductor and optical devices it is desired to maintain an undamaged thin film surface, which will be referred to as the frontside, while etching vias completely through the supporting substrate from the opposite side, hereafter referred to as the backside. If the front thin film surface does not have a strong resistance to the etch method being used to remove the backside substrate, the frontside thin layer will be damaged or destroyed.
One device which requires such an etching process is an infrared (IR) imaging array which is described in detail in (1) U.S. Pat. No. 4,080,532, Hopper, 3/1978; (2) U.S. Pat. No. 4,745,278, Hanson, 5/1988; (3) U.S. Pat. No. 4,792,681, Hanson, 12/1988; and (4) "LOW-COST UNCOOLED FOCAL PLANE ARRAY TECHNOLOGY", by Hanson, Beratan, Owen and Sweetser, presented Aug. 17, 1993 at the IRIS Detector Specialty Review.
Although the novel process to be described is not restricted to this type of IR imager, a brief description of the physical requirements clarifies the need for such processes.
An area imager may contain several hundred to tens of thousand individual picture elements (pixels). Each of these pixels consists of a capacitor (or resistor or another type of electronic element) that has a heat (IR intensity) sensitivity. Making use of the fact that the charge stored by a capacitor is proportional to the product of its terminal voltage and its capacitance, electronic circuitry can be attached to the two terminals of the capacitor based pixel to measure the intensity of the IR impinging on a specific pixel. Obstructions in the imaging field are removed and the electronic connections to these capacitors are simplified if one of the capacitor terminals is made common to all. Hundreds to tens of thousands of connections must still be made between the other isolated terminals of the capacitors and the electronic sensing circuitry. In addition, the pixel capacitors should be thermally isolated from each other while having one terminal connected to all the other common terminals.
The common connection to one side of the pixel capacitors consists of a frontside thin film. This may be a composite of a plurality of thin films which have been optimized with regard to IR absorbency, IR transparency, electrical conductivity, thermal resistivity, etc.. The thicker heat sensitive dielectric substrate in this case can be barium-strontium-titanate (BST) which is a ceramic perovskite material.
To leave the frontside thin film electrically conducting while isolating the pixels thermally, one may etch vias from the backside of the BST and stop the etch at the back surface of the frontside thin film. There has not yet been found a reactive type etch with sufficient selectivity that will etch the perovskite material and not harm the frontside thin film. Two of the few ways to remove BST is by the highly nonspecific methods of ion milling or laser ablation. Although at somewhat varying rates, most materials are etched by both of these methods. Neither the spatial uniformity nor the etch rate of ion milling or laser ablation can be controlled accurately enough to stop the backside etching of the substrate before damage occurs to the frontside film.
This invention describes methods to remove thick substrate material from the backside to form vias all the way to the back surface of the frontside material without frontside damage.